I have been breweing now for several years and have become lazy with some of the steps. I made an extreme lager and kegged and carbonnated it after three months of lagering without checking the FG. Regretably, it is too sweet and I suspect the high alcohol and sugar arressted complete fermentation. I immediately placed in "the cellar" to allow futher aging. My question is can this beer be salvaged with time or is it toast?

The OG was 1.082, and I am certain that the starting yeast (WLP Pilsen Lager) was overwhelmed by the strength. Since, in retrospect that is the probelm, my question is, can I salvage the beer that is now kegged and gassed. Can it be placed back in the carboy, allowed to decarbonate and finished with, perhaps, champagne yeast?

Otherwise, no need to put the beer back in a carboy. I'd release the keg pressure, pitch the yeast in the keg, pressurize it just enough to seal the lid again, as let the yeast ferment it out. You may want to check and make sure no excessive pressure is building up, but depending on how far the gravity needs to drop, you may just end up with a nicely naturally carbonated beer.